The present invention relates to a process for treating molten aluminum to remove hydrogen gas and non-metallic inclusions from the melt.
The term "aluminum" as used herein and in the appended claims includes pure aluminum and all aluminum alloys. Further the term "inert gas" used includes argon gas, helium gas, krypton gas and xenon gas on the Periodic Table and nitrogen gas which is inert to aluminum.
Molten aluminum before casting contains dissolved hydrogen gas and non-metallic inclusions, such as oxides of aluminum and magnesium, as undesirable impurities. Hydrogen gas and non-metallic inclusions, when present in molten aluminum, could produce defects in the ingots prepared from the melt and also in the products prepared from the ingot. Accordingly hydrogen gas and non-metallic inclusions must be removed from the molten metal.
Hydrogen gas and non-metallic inclusions are removed from molten aluminum usually by introducing an inert gas or chlorine gas into the molten metal in the form of bubbles. However, since the atmosphere contains water (in an amount of up to about 30 mg/liter in summer or up to about 5 mg/liter in winter in Osaka, Japan), aluminum and the water in the atmosphere react on the surface of the molten metal (2Al+3H.sub.2 O.fwdarw.Al.sub.2 O.sub.3 +3H.sub.2), giving rise to the problem that the resulting hydrogen penetrates into the melt. The surface of molten aluminum which is allowed to stand is usually covered with a compact aluminum oxide coating, so that the water in the atmosphere will not react with aluminum. Nevertheless, when a treating gas, such as an intert gas or chlorine gas, is forced into molten aluminum, the bubbles released to float on the surface of the melt disturb the surface and break the aluminum oxide coating over the melt surface, exposing the melt to the atmosphere at the broken portion. The water in the atmosphere then reacts with aluminum before a fresh oxide coating is formed at the broken portion, producing hydrogen gas and permitting the gas to penetrate into the melt.
Accordingly another process has been proposed in which a treating vessel of closed construction is used for containing molten aluminum (U.S. Pat. No. 3,870,511). With this process, an inert gas is filled into the vessel above the surface of the molten aluminum placed therein, and a treating gas is introduced into the melt while maintaining the gas atmosphere at a pressure higher than atmospheric pressure. This process, however, requires an expensive apparatus for holding the treating vessel closed. Further even if having a closed structure, the vessel inevitably permits ingress of some atmospheric air through the inlet for the molten metal or through a small clearance between the lid and the vessel main body. Our experiments have revealed that even when the water content of the atmosphere above the molten aluminum surface increases to as small a value as about 0.5 mg/liter owing to the ingress of air, the hydrogen resulting from the reaction between the water and the molten aluminum penetrates into the melt. The process therefore fails to achieve a satisfactory effect to remove hydrogen gas.
Furthermore, it is difficult for the conventional process to effectively remove hydrogen gas from a melt of aluminum having a high purity of not lower than 99.9 wt. %.